An object in static equilibrium is unable to move because all the forces acting on it compensate for one another. Static equilibrium is an important concept in the design of countless rigid structures, from the floor system of a house to an enormous suspension bridge, because these structures must maintain static equilibrium under all expected loading conditions.

A Balance of Forces

The basic condition for static equilibrium is that an object is not experiencing any type of motion, translational or rotational. An object in translational equilibrium is not traveling from one place to another, and an object in rotational equilibrium is not rotating around an axis. Translational equilibrium requires that the vector sum of all external forces is zero; in other words, the magnitudes and directions of external forces cancel each other out. Similarly, rotational equilibrium requires that all of the external torques cancel each other out. A torque is an influence that acts on the rotational motion of an object.

Static equilibrium is a valuable analysis tool: for example, if two forces are acting on an object that is in static equilibrium, that means they add up to zero. If you know the direction and magnitude of one of the forces, you can write an equation to determine the magnitude and direction of the unknown force.